Is this transformer any good for anything?

[38eighteen]

A friend called me to his work because they had a heap of stuff they were throwing out and he knew I was interested in electronics. I got a bunch of high wattage resisters, bridge rectifiers, power cords etc.

But the things I was most interested in were some nice big toroidal transformers. That was until I read the specs. I don't think they are any good for amplifiers, but I'd be really happy if someone showed me a way that I could use them as I got 4 of them!

I am in Australia so my mains power is nominally 240V. The transformers are:

PRI: 2 x 120V
SEC: 11V
SEC: 11.5V
SEC: 6V

382VA !!

Can I somehow combine the two PRI to match my 240V mains and combine the three SEC to give me 28.5V?

Thanks,

Brian

[Elvee]

Hi

IF and only if all secondaries have the same current rating (same wire gauge), you can put them in series and get the full 382VA; otherwise, you'll have to derate, on the base of the weakest winding.
LV

[AndrewT]

Hi,
where did 382VA come from?
What are the solid core secondary wire diameters?

[38eighteen]

Originally posted by Elvee
IF and only if all secondaries have the same current rating (same wire gauge), you can put them in series and get the full 382VA;

I found the markings (very faint) on the cable and tracked it down. The secondaries are all "TYCAB HT90 0.6/1.0 KV". From the manufacturer's web site, this is approx. AWG 13 (50 x 0.25mm conductors) rated at 20A.

Originally posted by AndrewT
where did 382VA come from?

It is printed on the side.

Originally posted by AndrewT
What are the solid core secondary wire diameters?

See above.

I've been to the manufacturer's web site and it does not seem to be one of their standard configurations.

So, it looks like I can just wire the primaries and secondaries (http://www.hammondmfg.com/5CHook.htm) in series and have 28.5V?

Thanks,

Brian

[megajocke]

Do you mean that each secondary wire has 50 pieces of 0.25mm conductors? You need the wire size of the winding, those you have measured are just the leadout wires if they are multistranded. Measuring resistance with a milliohmmeter could be an alternative to disassembling the transformer.

[38eighteen]

Originally posted by megajocke
Do you mean that each secondary wire has 50 pieces of 0.25mm conductors?

Yes.

Originally posted by megajocke
You need the wire size of the winding, those you have measured are just the leadout wires if they are multistranded. Measuring resistance with a milliohmmeter could be an alternative to disassembling the transformer.

I don't have a milliohmmeter. My cheapish DMM says that all secondaries have 0.0 ohm resistance

I have at least 4 (and maybe 8 or 10) of these so I am happy to experiment a bit with one. Is it as simple as pealing off some of the tape and measuring the diameter of the winding? Is the measurement I take with a manual (ie. non digital) vernier calliper going to be accurate enough to mean anything? Can I just tape it back up and use it?

Thanks,

Brian

[Elvee]

Originally posted by 38eighteen


Yes.



I don't have a milliohmmeter. My cheapish DMM says that all secondaries have 0.0 ohm resistance

You can force a current, say 0.1A or 1A, using an external DC source and a resistor and measure the voltage drop. Take care to make the measurement in 4 wire mode, i.e. establish the forcing circuit and measure the drop directly on the transformer's wires.

[AndrewT]

Hi,
unwind the insulating tape, taking note of how each winding is overlapped to ensure continuity of the insulation.
You have to rewind it in a similar manner. Keep an even tension on the tape stretching it slightly but not enough to break it. Try to get it as flat as possible to cover as much area as possible. In the middle of the toroid the overlap will be around six times, but at the outer perimeter you are looking for 55% overlap to guarantee at least two thicknesses at every location.

Now to the wires.
Find the junctions between the leadouts and the windings. They are usually grouped together but spaced apart and well protected from abrasion/vibration to prevent insulation failure. If you are unsure of your prowess, it may be wise to leave these junctions undisturbed.
Just measure a few of the exposed wires that you can "grip" with the calipers. Visually check that all the windings are the same diameter, and note there may be a subtle colour change between the windings since they have come off different spools.

Using your 28.5Vac and 382VA indicates output current of 13.4A.
This will probably use 4.3sqmm of winding and be about 2.4mm diameter. No, it won't, it will more likely be bifillar wound using 1.6 or 1.7mm diameter wire.
If it is bifillar wound you have another option.

[38eighteen]

Originally posted by AndrewT
No, it won't, it will more likely be bifillar wound using 1.6 or 1.7mm diameter wire.
If it is bifillar wound you have another option.

The tape is clear and I can see that there are no 2.4mm wires inside. (Well, at least visible). They do look more like 1.6 or 1.7. And, they seem to be roughly in pairs.

Any unwinding and measuring is probably going to destroy the transformer as it has a plastic mounting plate moulded onto the base and this comes up about 3/4 of the way through the centre of the toroid. I will check my "total stock numbers" before deciding if I want to sacrifice one for this.

I have written to the manufacturer with the batch number and asked for information. I don't like my chances, especially at this time of year, but you never know...

Thanks,

Brian

[AndrewT]

Hi,
I have heard of builders removing that centre plug.
Methods adopting vary but drilling, and heating can be used.
I tried the drilling method once and after drilling a circle of 6mm holes the plug just pushed out, little adhesion between the black potting and the insulating tape.

Bifillar winding gives the option to split the parallel wires and terminate each of them with leadouts. Do lots of measuring to ensure this is what you have. I had a trifillar 25A toroid, split it into 3 secondaries and added a fourth to give 4 equal voltage secondaries. Ideal for a two channel semi-monoblock assembly.